Terror secrets of Bin Laden's mansion: Al Qaeda was planning to derail U.S. trains on 10th anniversary of 9/11

A treasure trove of information about planned terror attacks aimed at killing hundreds of innocent people has been found in the material seized at Osama bin Laden's compound.

The swathes of evidence discovered on computers, DVDs and documents recovered by U.S. Navy SEALs in Sunday's shoot out show that the terror mastermind was still keen on so-called 'soft targets' like trains and planes.

U.S. officials are calling the find the largest potential intelligence coup of the post 9/11 era.

The only plot that looked near to being ready to carry out was one to derail a train on the 10th anniversary of the 9/11 terror attacks, according to a new bulletin issued tonight by the FBI and the Department of Homeland Security, which was obtained by ABC News.

Uncovered: U.S. officials analysing the mountain of evidence recovered from Osama bin Laden's lair discovered that Al Qaeda was planning to derail a train on the 10th anniversary of the 9/11 terror attacks

Chillingly the plan, which was found on a set of handwritten notes contained in a book, had been in place as early as February of last year, but there was no evidence to suggest it would happen in New York, according to an official.

The book of notes discussed 'attacking America during symbolic times and using mass transit',

an administration official told the Wall Street Journal.

Other material gathered from the compound suggests that Al Qaeda was also looking to strike Washington, New York, Los Angeles and Chicago on dates like July 4, Christmas and the opening day of the UN, officials said.

'As of February 2010, Al Qaeda was allegedly contemplating conducting an operation against trains at an unspecified location in the United States on the 10th anniversary of September 11, 2001,' the bulletin reads.

'As one option, Al Qaeda was looking into trying to tip a train by tampering with the rails so that the train would fall off the track at either a valley or a bridge.'

No chances: An armed New York City police officer stands guard near Ground Zero, ahead of President Obama's visit

Vigilance: In New York City armed Metropolitan Transportation Authority police officers stand guard at Grand Central Station, left, and Port Authority, right

Meanwhile... Despite the potential terror plot details being revealed Barack Obama still made a visit to Ground Zero to lay a wreath for victims of the 9/11 terror attacks

The Department of Homeland Security (DHS) has decided not to issue a National Terrorism Advisory System alert in wake of the new evidence.

In a statement, DHS press secretary Matt Chandler, said the department had instead sent out a message to its rail partners about the potential al Qaeda plot and remained at a heightened state of vigilance.

Mr Chandler added the Transportation Security Administration would be sending a bulletin to its rail sector stakeholders in due course.

'We have no information of any imminent terrorist threat to the U.S. rail sector, but wanted to make our partners aware of the alleged plotting,' Mr Chandler told ABC News.

The Navy SEALs recovered five computers, 10 hard drives and more than 100 storage devices including DVDs and memory sticks, U.S. officials told the Wall Street Journal.

A CIA task force is said to have done a preliminary sweep of the material, described by one official as 'scanning the treetops for the most immediate intelligence,' for imminent plots.

Tourist attraction: A Pakistani woman poses for a picture with her daughter in front of the locked gates at Osama bin Laden's compound in Abbottabad

Evidence: Computers, DVDs and documents scattered around the rooms of the compound were taken by U.S. Navy Seals and are being analysed for potential Al Qaeda plots

Lair: It is thought that Bin Laden had lived at the compound for five years and his wife, Amal al-Sadah, revealed to Pakistan security officials the couple had not come out of the room where he was shot in that time

They are also looking for any clues about Bin Laden's right-hand man Ayman al-Zawahiri, who is expected to become the next Al Qaeda leader, and controversially, whether members of the Pakistani government knew about Bin Laden's hideout and were helping to conceal him.

It is likely to take weeks for the task force to translate and analyse all the data, the official added.

In light of the news the DHS has taken a 'number of actions' to step up security. These include additional officers based at airports. It said it is also reviewing preventative measures for potential al Qaeda targets.

Today authorities were taking no chances. At Grand Central Station and Port Authority in New York City armed Metropolitan Transportation Authority police officers stood guard as hundreds of commuters filed past.

Armed guards were also present in the city for Barack Obama's scheduled visit to Ground Zero to lay a wreath for the victims of the 9/11 terror attacks.

He took part in a moment of silence and also spent time meeting the families of victims before having lunch at Engine Company 54, which lost 15 firefighters in the attacks.

'It's some comfort, I hope, to all of you to know that when those guys took those extraordinary risks going into Pakistan, that they were doing it in part because of the sacrifices that were made in the States,' Mr Obama said to the firefighters.

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The potential rail attack is just one of a variety of concerns being discussed by the DHS and regional authorities but tonight it is not yet clear what other areas the terror group were - or indeed are - still targeting.

The evidence also suggests that Bin Laden was still very much involved in planning and approving al Qaeda plots despite being on the run for ten years.

One U.S. official told ABC News the documents that had been analysed so far, including schematics and websites, showed a variety of plots Al Qaeda had been hatching.

These he described as 'aspirational' plots that were by no means ready to carry out.

Officials have long been concerned that Al Qaeda might try to carry out attacks on the U.S. rail system.

In 2008, U.S. authorities warned of a possible Al Qaeda threat to transit systems in and around New York City over the Thanksgiving holiday weekend.

Last year, an Afghan immigrant pleaded guilty in New York to plotting a suicide bombing campaign on Manhattan's subway system in what U.S. authorities described as one of the most serious threats since the September 11 attacks.